BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to nuclear magnetic resonance (NMR) spectroscopy and,
more particularly, to NMR spectrometer probes which can generate a magnetic field
gradient along the axis of a sample being rotated at the "magic angle."
2. Description of the Related Art
[0002] All atomic nuclei with an odd atomic mass or an odd atomic number possess a nuclear
magnetic moment. Nuclear magnetic resonance (NMR) is a phenomenon exhibited by this
select group of atomic nuclei (termed "NMR active" nuclei), and is based upon the
interaction of the nucleus with an applied, external magnetic field. The magnetic
properties of a nucleus are conveniently discussed in terms of two quantities: the
gyromagnetic ratio (y); and the nuclear spin (I). When an NMR active nucleus is placed
in a magnetic field, its nuclear magnetic energy levels are split into (21 + 1) non-degenerate
energy levels, which are separated from each other by an energy difference that is
directly proportional to the strength of the applied magnetic field. This splitting
is called the "Zeeman" splitting and is equal to yhH
o2π, where h is Plank's constant and H
o is the strength of the magnetic field. The frequency corresponding to the energy
of the Zeeman splitting (w
o=yH
o) is called the "Larmor frequency" and is proportional to the field strength of the
magnetic field. Typical NMR active nuclei include
1H (protons),
13C,
19F, and
31p. For these four nuclei I =1/2, and each nucleus has two nuclear magnetic energy
levels.
[0003] When a bulk sample containing NMR active nuclei is placed within a magnetic field,
the nuclear spins distribute themselves amongst the nuclear magnetic energy levels
in accordance with Boltzmann's statistics. This results in a population imbalance
between the energy levels and a net nuclear magnetization. It is this net nuclear
magnetization that is studied by NMR techniques.
[0004] At equilibrium, the net nuclear magnetization is aligned parallel to the external
magnetic field and is static. A second magnetic field perpendicular to the first and
rotating at, or near, the Larmor frequency can be applied to induce a coherent motion
of the net nuclear magnetization. Since, at conventional field strengths, the Larmor
frequency is in the megahertz frequency range, this second field is called a "radio
frequency" or RF field.
[0005] The coherent motion of the nuclear magnetization about the RF field is called a "nutation."
In order to conveniently deal with this nutation, a reference frame is used which
rotates about the z-axis at the Larmor frequency. In this "rotating frame" the RF
field, which is rotating in the stationary "laboratory" reference frame, is static.
Consequently, the effect of the RF field is to rotate the now static nuclear magnetization
direction at an angle with respect to the main static field direction. By convention,
an RF field pulse of sufficient length to nufate the nuclear magnetization through
an angle of 90°, or π/2 radians, is called a "π/2 pulse."
[0006] A π/2 pulse applied with a frequency near the nuclear resonance frequency will rotate
the spin magnetization from an original direction along the main static magnetic field
direction into a plane perpendicular to the main magnetic field direction. Because
the RF field and the nuclear magnetization are rotating, the component of the net
magnetization that is transverse to the main magnetic field precesses about the main
magnetic field at the Larmor frequency. This precession can be detected with a receiver
coil that is resonant at the precession frequency and located such that the precessing
magnetization induces a voltage across the coil. Frequently, the "transmitter coil"
employed for applying the RF field to the sample and the "receiver coil" employed
for detecting the magnetization are one and the same coil.
[0007] In addition to precessing at the Larmor frequency, in the absence of the applied
RF energy, the nuclear magnetization also undergoes two relaxation processes: (1)
the precessions of various individual nuclear spins which generate the net nuclear
magnetization become dephased with respect to each other so that the magnetization
within the transverse plane loses phase coherence (so-called" spin-spin relaxation")
with an associated relaxation time, T
2, and (2) the individual nuclear spins return to their equilibrium population of the
nuclear magnetic energy levels (so-called "spin-lattice relaxation") with an associated
relaxation time, T
1.
[0008] The nuclear magnetic moment experiences an external magnetic field that is reduced
from the actual field due to a screening from the electron cloud. This screening results
in a slight shift in the Larmor frequency (called the "chemical shift" since the size
and symmetry of the shielding is dependent on the chemical composition of the sample).
[0009] Since the Larmor frequency is proportional to the field strength, it is generally
desirable to insure that the main magnetic field and the RF magnetic field are spatially
homogeneous, at least in the sample area, so that all parts of the sample generate
an NMR signal with the same frequency. However, there are some known applications
of NMR techniques for which it is desirable to establish a magnetic field gradient
across the sample: examples of such applications include NMR imaging, molecular diffusion
measurements, solvent suppression, coherence pathway selection and multiple-quantum
filters.
[0010] A conventional method of applying such gradients is to use special gradient coils
in addition to the coils which generate the main static field and the coils which
generate the RF magnetic field. These special coils are located in the NMR probe and
generate a magnetic field gradient called a B
o gradient which has at least one field component that has a direction parallel to
the main static field direction, but has an amplitude which . varies as a function
of spatial position. All of the aforementioned NMR applications have been demonstrated
utilizing a B
0 gradient. The coils which generate the B
o gradients along the Cartesian axes are well-known.
[0011] Many samples of solids or gels display rather broad NMR resonances when measured
via liquid state NMR methods since the molecules are not free to tumble rapidly and
isotropically. These additional broadenings arise from dipole-dipole interactions
between spins, the anisotropy of the chemical shift and local variations in the magnetic
susceptibility. Magic angle sample spinning (MAS) is a well-known means of restoring
the spectra to a seemingly high resolution result by introducing a physical rotation
of the sample as a whole about the so-called magic angle, θ
m, to the static field direction, where cos θ
m =
. This angle corresponds to the bisector of a cube (along the [1,1,1] direction relative
to an x,y,z Cartesian coordinate system), and rotation about this axis creates an
equal weighting of evolution for the x, y, and z directions, averaging out the local
variations. Provided that the spinning rate is fast compared to the line width, sharp
resonances are observed in MAS experiments. Spinning rates of from 2 to 10kHz are
routinely achieved in MAS probes.
[0012] For certain solid state imaging experiments, gradient coils have been used with MAS
probes to create a rotating imaging reference frame which, relative to the sample,
appears stationary. This differs from the conventional reference frame which is actually
fixed, and within which the sample is rotating relative to the reference frame at
the above-mentioned spinning rate. In all of the approaches to creating such a modified
reference frame, the gradient fields were caused to rotate synchronously with the
sample by modulating the currents through specially designed gradient coils. In order
to use three gradient sets to generate a ∂Bz/∂θ
m field, a full set of three gradient coils is required, each with its own audio amplifier.
The coils are driven by a carefully tuned oscillator circuit that must be phase-locked
to the position of the sample. This high degree of complexity is essential for imaging
when all three spatial axes are used, but is more than needed for spectroscopy.
[0013] In another prior art design, a set of gradient coils has been implemented that creates
a gradient field oriented along the magic angle by creating a complex current distribution
on a cylinder oriented along the static field. This arrangement was proposed to avoid
interferences from dipolar demagnetizing fields and was not designed to operate with
a MAS probe. It has the gradient coils wrapped on a cylinder oriented along the main
magnetic field direction. This method suffers from a number of difficulties. First,
the arrangement would have to take up space which is required for the MAS stator.
Secondly, the gradient coil must be precisely aligned with the spinner axis and, with
the gradient coil and the stator as two separate pieces, alignment of the two pieces
must be performed for each probe. Also, the gradient coil can interfere with the ejection
of the sample container.
[0014] The document of Bowtell and Peters, "Magic-Angle Gradient Coil Design"; Journal of
Magnetic Resonance, series A, volume 115, pp. 55-89 (1995) discloses a gradient coil
design having a closed current flow structure along a cylindrical envelope whose axis
is aligned along the z-axis. This closed structure renders of the application of this
gradient coil design to magic angle spinning extremely difficult since the stator
and rotor must be oriented at the magic angle with respect to the z-axis thereby requiring
such gradient coils to have a very large geometry.
[0015] When implementing gradient spectroscopy experiments in a MAS probe, it would be desirable
to have a gradient design in which the gradient is oriented such that the z-component
of the magnetic field increases along the axis of the spinner, and the gradient magnetic
field is uniform in the planes perpendicular to the spinner axis. It would also be
desirable to have a gradient field which is obtained for a DC current through the
gradient winding, and for which no synchronization to the spinner motion is necessary.
Furthermore, it would be advantageous if such a design was compatible with the mechanical
layout of prior-art MAS stator designs, did not interfere with sample insertion/ejection,
and had a gradient strength of conventional strength (on the order of
SUMMARY OF THE INVENTION
[0016] The present invention provides a gradient field generator for an NMR probe which
consists of a plurality of straight line conductive segments surrounding a stator
within which a sample container is housed. The conductive segments are parallel to
one another, and perpendicular to a plane containing a rotation axis about which the
sample container is rotatable. Preferably, the conductive segments are equidistant
from a center of the sample volume, arranged in a cylindrical distribution around
the sample container. The currents through the conductive segments approximate a cylindrical
current distribution, and provide a gradient magnetic field across the sample volume
wherein the field strength of said gradient magnetic field varies in a substantially
linear manner along the direction of the rotation axis.
[0017] In a preferred embodiment of the invention, the conductive segments are formed by
wrapping a conductive material about the body of the stator. The conductive material,
preferably wire, is wrapped along the outer surfaces of the stator in the locations,
directions and number of turns which allow a DC current to be passed through the conductive
material which generates the desired currents in the appropriate conductive segments.
Small holes may also be drilled in the body of the stator, and the conductive material
passed through these holes to allow wrapping of the conductive material in the desired
locations. Magnetic shielding may also be used around the conductive sections to minimize
eddy currents in surrounding materials.
[0018] Once a desired number of conductive segments is selected, one method of determining
the appropriate currents is to specify the Jacobian describing the magnetic field
in the three-dimensional space surrounding the sample container. That Jacobian has
the form:
where z is the direction of the static magnetic field of the probe, the y-z plane
is that in which the stator is adjusted for the magic angle, and x is the direction
to which the conductive segments run parallel.
[0019] For the application of MAS spectroscopy, the Jacobian has the following values:
[0020] Given this constraint, the number of solutions may be further reduced by restricting
the locations of the conductive segments to those falling within a cylindrical pattern
surrounding the stator. With the only independent variable then being the angle between
the position of each conductive segment in the x-y plane and the z-axis, and knowing
the desired number of conductors, positions for the conductive segments may be selected
along the cylinder surrounding the stator which provide the desired gradient field,
and which require currents that are most easily provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
FIG. 1 is a partial cross-sectional side view of a MAS probe and stator having a gradient
magnetic field generator according to the present invention.
FIG. 2 is a partial cross-sectional top view of the MAS probe and stator of the embodiment
shown in FIG. 1.
FIG. 3 is an isometric view of the MAS stator of the embodiment of FIG. 1 showing
the straight line conductors used to generate the desired gradient magnetic field.
FIG. 4. is a side view of the stator shown in FIG. 3.
FIG. 5. is a graphical depiction of the current distribution provided by the straight
line conductors shown in FIG. 3.
FIG. 6. is a graphical depiction of the gradient field variation for the current distribution
shown in FIG. 5.
FIG. 7 is a graphical depiction of the current distribution provided by an alternative
embodiment of the invention which uses eight straight line conductive segments.
FIG. 8 is a graphical depiction of the gradient field variation for the current distribution
shown in FIG. 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] Shown in FIG. 1 is a cross sectional side view of a portion of the probe housing
10 and stator 12 of a nuclear magnetic resonance (NMR) probe according to the present
invention. The probe is intended for use with solid state sample material, and therefore
rotates the sample at the well-known "magic angle" relative to the direction of the
main magnet field. Provided with FIG. 1 is a two-dimensional Cartesian coordinate
system showing the selected designations of direction relative to the probe. In this
embodiment, the probe is coaxial with the cylindrical magnet bore and, therefore,
the main magnetic field is in the direction of the z-axis. The angular position of
the stator 12 within the y-z plane is thus set at the "magic angle" (approximately
54.70 relative to the z-axis).
[0023] The probe of FIG. 1 is capable of generating a gradient magnetic field oriented along
the magic angle. For clarity, a broken line Cartesian coordinate axis set is overlaid
on the y-z coordinates shown in FIG. 1. The broken line axes are labeled z' and y',
and represent the y-z plane rotated by the magic angle. Thus, the desired direction
of the gradient field is the z' direction. This rotated coordinate system will simplify
the description of the present invention provided below.
[0024] The probe of FIG. 1 generates the gradient field using straight line conductive segments
oriented in the x-direction. The conductive segments (hereinafter "conductors") are
located relative to the stator 12, and provided with the necessary current distribution
so as to generate the desired gradient field. The conductors are referred to herein
generally using the reference numeral 14, with individual conductors being identified
by the designations 14a, 14b, etc. In the embodiment of FIG. 1, four conductors are
used, but it will become apparent that any number of conductors which is a multiple
of four can be used to construct the probe.
[0025] The conductors 14 are placed adjacent to the surface of the stator 12, and are physically
oriented in the desired position for the generation of the gradient magnetic field.
The position of the conductors also takes into account the location of sample ejection
port 15, such that the conductors do not inhibit loading and ejection of the sample
container. The position and orientation of the conductors 14 is relative to an active
sample volume 16 of sample container, or "spinner", 18. In FIG. 1, the spinner 18
and sample volume 16 are shown in outline form to indicate their location. As mentioned
above, the currents passed through conductors 14 generate a desired gradient magnetic
field in the y and z direction throughout the active volume 16. In the preferred embodiment,
the conductors 14 are parallel to one another and equidistant from the center of the
sample volume 16. This is demonstrated in FIG. 1 by the circle 19 shown in the y-z
plane, which is centered about the center of the active volume 16, and which intersects
each of the four conductors 14. Circle 19 demonstrates the cylindrical distribution
of the straight line conductors 14 about the center of the sample volume. As discussed
below, this orientation of the conductors simplifies the selection of currents to
be used for the four conductors 14. Also shown (as a broken line) in FIG. 1 is cylindrical
magnetic shielding 17. The shielding may be used to prevent the magnetic fields generated
by the conductive sections 14a-14d from inducing eddy currents in surrounding materials.
Those skilled in the art will recognize, however, that a complete cylindrical shield
can not be used if there is to be ejection of the sample container through ejection
port 15.
[0026] In the preferred embodiment, the conductors 14 are portions of a conductive material
which is wrapped about the stator 12, typically a wire. FIG. 2 is a two-dimensional
schematic view of the stator within probe housing 10 relative to the x-y plane, with
conductors 14 having been formed from wire wrapped about the stator in two primary
loops 20, 22. The two loops are each substantially rectangular in shape, and each
consists of one or more wire strands. As shown, each loop has two linear portions
14 parallel to the x-axis (one to a first side of the stator and the other to the
opposite side). Loop 20 includes conductors 14a and 14b, while loop 22 includes conductors
14c and 14d. Each loop also has two substantially linear portions 24 residing perpendicular
to the x-axis. These linear portions are referred to generally herein by reference
numeral 24, and are also referred to individually by reference numerals 24a. 24b,
etc.
[0027] Because the linear portions 24 are oriented partially in the z-direction, and are
far removed from the sample area, they contribute very little to the gradient field.
In the preferred embodiment of the invention, the loops 20,22 are actually part of
the same conductor which is wrapped about the stator to form loop 20 in one position,
and to form loop 22 in another position. The conductor has two sections (not shown
in FIG. 2) which connect the two loops 20, 22.
[0028] FIG. 3 is an isometric view of stator 12 showing the conductive loops 20, 22, consisting
of linear conductive segments 14a-14d and linear segments 24a-24d, wrapped around
the stator 12. A side view of the stator 12 is also shown in FIG. 4, the orientation
of which is indicated by the section line IV-IV in FIG. 3. Referring again to FIG.
3, conductive segments 26, 28 run perpendicular to the x-axis and interconnect the
loops 20,22. To assist in understanding the arrangement shown in the figure, the hidden
outer surfaces of the stator are shown using dashed lines, as are the hidden portions
of the loops. Also shown in broken lines is sample spinner 18. The depiction of the
sample spinner 18 in FIG. 3 is to indicate the location of the spinner 18 relative
to the stator 12 and the conductors 14. Those skilled in the art will recognize that
there is additional support structure for the spinner 18 within the body of the stator
12 which is not depicted in FIG. 3.
[0029] Current through the conductive material surrounding the stator 12 follows a specific
path for the stator of FIG. 3. The current enters through one of two conductors which
make up twisted conductor pair 30. The current then travels around the loop 20, passing
first through conductor 14a, through conductor 24a, back through conductor 14b, and
then returning through conductor 24b to the point at which the current first entered
the loop. Depending on the position and gauge of the conductive material, the level
of current used, and the desired magnetic field, the loop 20 may consist of a number
of adjacent turns following the same conductive path as shown for conductors 14a,
24a, 14b and 24b, eventually returning to the point at which the current first enters
the loop 20. After the desired number of turns about loop 20, the current passes from
loop 20 to loop 22 along conductor 26.
[0030] The current traveling along conductor 26 passes to conductor 24d. From conductor
24d, the current travels through conductor 14d, through conductor 24c, and returns
along conductor 14c to the point at which it entered the loop 22. As with the loop
20, the loop 22 may actually consist of a number of turns of the conductive material
wrapped about the stator 12, following the same path as described above, and returning
to the point at which the current entered the loop 22. The current then travels along
conductor 28, following the opposite path as the current flowing through conductor
26. The current then follows the second of the two conductors of twisted pair 30,
returning to the source of the current, indicated schematically in FIG. 3 as current
source 32. Thus, with the appropriate setting on current source 32, the desired currents
for each of the four linear conductors 14 are generated.
[0031] In general, the use of a single current source to power all of the conductive segments
is preferred. When using a single current source, currents of different magnitudes
in different segments may be created by using a different number of turns for the
different segments. Thus, while there is only one power source, a ratio of turns between
various segments establishes a desired ratio of currents.
[0032] A straightforward and general approach to determining the appropriate currents through
the conductive segments 14 is to first specify the Jacobian that describes the variation
of the magnetic field and then to translate this into an ideal current distribution.
In general, the Jacobian for the three-dimensional space in the vicinity of the probe
has the form:
[0033] In NMR experiments the dynamics of the spin system are only sensitive to static fields
along the same direction as the externally applied magnetic field, which is typically
designated the z-direction. Therefore, for an NMR gradient experiment, it is sufficient
to specify the three components,
∂
Bz/ ∂x, ∂
Bz/ ∂y, ∂
Bz/ ∂z
[0034] For the present invention, a magnetic gradient field is desired that increases along
the direction of the magic angle. The magic angle has been located within the y-z
plane, although the x-z plane could have just as well been selected, since the NMR
experiments in question have cylindrical symmetry. The spinner axis (which is aligned
with the magic angle) is therefore along the vector,
[0035] In the preferred embodiment, the z-component of the gradient field is to increase
also in the direction of above vector, and to be uniform otherwise. Thus, the spatial
variation in the z-components of the field must satisfy the conditions,
∂
Bz/ ∂x = 0, ∂
Bz/ ∂y= sin Θ
m, ∂
Bz/ ∂z = cos Θ
m .
[0036] At the magic angle, sin Θ
m =
and cos Θ
m=
. These conditions obviously do not completely define the Jacobian. Indeed, there
are an infinite number of solutions to Maxwell's equations that are consistent with
the values given above for these three components. However, in the present invention,
the available solutions are limited by the constraint of having the conductive segments
located within the available confines of the probe. Thus, the present invention makes
use of straight line conductive segments 14 which fit within the leftover space within
the probe housing. Since the stator 12 is already oriented within the y-z plane (i.e.
relative to the x-axis) it is advantageous to orient the conductive segments parallel
to the x-axis. This is demonstrated in FIGS. 1-4, in which the conductive segments
14 are shown relative to stator 12.
[0037] The alignment of the conductors parallel to the x-axis simplifies determination of
the values of the Jacobian for the desired gradient field since,
∂
Bx/ ∂x = ∂
Bx/ ay = ∂
Bx/ ∂z = ∂
By/ ∂x = ∂
Bz/ ∂x = 0
[0038] Under these additional constraints, the following Jacobian is the only one that is
consistent with Maxwell's equations:
[0039] With Θ
m being equal to the magic angle, this Jacobian may be rewritten as:
[0040] The above Jacobian uniquely defines the magnetic field. However, a distribution of
currents that generates this field is also required. For the geometry specified here,
one simple solution is to define a distribution of current over the surface of a cylinder
oriented along the x-axis. Since the currents are restricted to being along the cylinder
surface, then there is only one independent variable, the angle between the current
segment on the cylinder and the z-axis. Other distributions may also be used within
the context of the invention, but the cylindrical distribution is favored for its
simplicity, and because it minimizes the introduction of higher order harmonics into
the gradient field.
[0041] While in principle the continuous current distribution described above solves the
problem, practical solutions are composed of discrete current distributions. Also,
the continuous current distribution would interfere with the loading of the sample
into the stator. Thus; the arrangement of straight line segments 14 is used in the
present invention to provide the desired current distribution. This distribution preserves
the symmetry of the above continuous current distribution while fulfilling the six
points of the desired gradient field. These points are: 1) only DC currents are used;
2) arrangement is compatible with existing stators; 3) no interference with sample
ejection; 4: a strong gradient field is provided; 5) arrangement is compatible with
existing gradient amplifier drivers; and 6) the construction is relatively easy to
manufacture.
[0042] FIG. 5 is a graphical depiction of the current distribution for the embodiment of
FIGS. 1-4 relative to the y-z plane. As shown, the location of the currents (and hence
the location of the straight line conductive segments) is shifted rotationally by
approximately 17.35°. This rotational position is necessary, but different radii of
the conductors about the sample may be used. The currents in this embodiment are identified
in the figure by reference numerals 34a, 34b, 34c and 34d. Relative to the embodiment
of FIGS. 1-4, current 34a is that which flows through conductor 14a, current 34b flows
through conductor 34b, current 34c flows through conductor 14c, and current 14d flows
through conductor 34d. The normalized magnitude of the currents for this distribution
is also shown, being +1 for currents 34b and 34c, while being -1 for currents 34a
and 34d. This distribution allows for the simple, wrapped-conductor arrangement shown
in FIGS. 1-4, particularly in light of the fact that the magnitude of each of the
currents is the same.
[0043] FIG. 6 is a graphical depiction of the gradient field distribution for the preferred
embodiment of the invention (i.e. for the currents shown in FIG. 5). FIG. 6 has a
horizontal axis along the y' direction which has a millimeter scale. The vertical
axis is along the z' direction, and has a scale of magnetic field strength. The units
for the vertical axis are arbitrary. However, the graph is overlaid with horizontal
lines each indicative of magnetic field strength for a particular value of z'. The
extension of these lines in the horizontal direction shows the slight variations in
the gradient magnetic field strength in the y'-direction, for each of a number of
different values of z' (i.e., z'=1, z'=2, etc.). These variations are due to the use
of the four straight line conductors 14 to generate the gradient field. However, as
shown, the linearity of the field lines is quite good, and acceptable for most experimental
purposes.
[0044] An alternative embodiment of the invention is shown in FIG. 7. Like FIG. 5, FIG.
7 is a graphical depiction of the currents used to generate a desired gradient magnetic
field. As shown, this embodiment uses eight currents rather than four. The currents
are distributed in a cylindrical pattern, equidistant from the center of the sample
volume. The magnitudes of the currents are found using the same method described above
for the four-current embodiment, and are shown in FIG. 7. This distribution of currents
leads to a gradient magnetic field indicated by the graphical depiction in FIG. 8.
This representation is in the same form as that of FIG. 6 and, as shown, the use of
eight conductors provides a slightly better linearity than the use of four conductors.
However, the four-conductor embodiment is sufficiently linear for most purposes, and
is preferred due to its ease of construction. Those skilled in the art will recognize
that any number of conductors which is a multiple of four may be used.
[0045] Because it is the relative currents in the conductive sections which provides the
desired gradient across the sample, it will be apparent to those skilled in the art
that it is not necessary that the specific applied currents be DC currents, as long
as the relative magnitudes of the currents of the different section remains the same.
Thus, alternating currents could also be used, as long as the currents in the different
sections change proportionally, and are in phase with one another. However, for the
preferred embodiment taught herein, DC currents are preferred.
[0046] Although only a few illustrative embodiments of the present invention have been disclosed
in the discussion above, other modifications and enhancements will be apparent to
those skilled in the art, which modifications and enhancements are intended to be
covered by the claims set forth below. For example, any distribution of currents that
preserves the symmetry of the above described continuous current distribution will
generate the desired magic angle gradient field.
[0047] A gradient magnetic field generator is provided for generating a spatially varying
gradient magnetic field for use with a nuclear magnetic resonance spectroscopy probe
having a rotatable sample container. The gradient field generator has a plurality
of straight line conductive segments which lie parallel to one another and perpendicular
to a plane within which lies a rotation axis about which the sample container rotates.
The straight line conductive segments each conduct a current which generates a component
of the overall gradient magnetic field. The conductive segments preferably lie in
a cylindrical distribution about a stator within which the sample container is rotated.
The appropriate currents for the conductive segments may be determined by finding
a solution for the Jacobian which defines the magnetic field variations in the three-dimensional
space of the stator. Finding an appropriate solution is simplified by presuming the
cylindrical 'distribution of conductive segments and allowing restriction due to the
size and shape of the stator, and the physical space between the stator and an inner
surface of the probe housing.
1. Nuclear magnetic resonance angle spinning (MAS) spectroscopy for analyzing a sample
contained within a sample container, the apparatus comprising:
a static magnetic field generator for generating a static magnetic field which influences
the sample, the static magnetic field being oriented in the z-direction; and
a stator for housing the sample container and allowing rotation of the sample container
therein about a rotation axis having an angle relative to the z-direction of substantially
θm = cos-1 √1/3;
characterized by
a gradient magnetic field generator comprising a plurality of straight line conductive
sections each of which contributes to a gradient field when one of a plurality of
respective predetermined currents is passed through it, the straight line conductive
sections being parallel to one another and perpendicular to a plane containing said
rotation axis and the main magnetic field direction, wherein said predetermined currents
generate a gradient field, wherein the field strength of the gradient field in the
z-direction changes linearly along said rotation axis.
2. Apparatus of claim 1, wherein said predetermined currents are direct (DC) currents.
3. Apparatus according to claim 1 or 2, wherein the conductive sections are all part
of a single conductor.
4. Apparatus according to claim 3, wherein the single conductor is wound about the stator.
5. Apparatus according to anyone of the preceding claims, wherein the conductive sections
are arranged in a cylindrical distribution such that they are substantially parallel
to and equidistant from an axis passing through the sample, wherein the axis passes
through the center of the sample volume, and wherein a normalized magnitude of each
of said predetermined currents is substantially equal to cos 2 (θ - 45° + θm/2) where θ is an angular position of a given conductive section relative to the z-direction.
6. Apparatus according to anyone of the preceding claims further comprising magnetic
shielding encompassing the conductive segments.
7. Apparatus according to anyone of the preceding claims, wherein the conductive sections
are physically supported by the stator and the stator further comprises an ejection
port through which the sample container may be ejected from the stator, the conductive
sections being positioned such that they do not inhibit ejection of the sample container
through the sample port.
8. Apparatus according to claim 7, wherein the predetermined current through each of
the conductive sections is unsynchronized with the rotation of the sample.
9. Apparatus according to anyone of the preceding claims, wherein the conductive sections
comprise two conductive sections, each having a current of i1, located along the cylindrical distribution, respectively, at 0 ≈ 17.65° and θ ≈
197.65°, and two conductive sections, each having a current or -i1, located along the cylindrical distribution, respectively, at θ ≈ 107.65° and θ ≈
287.65°.
10. Apparatus according to anyone of the preceding claims, wherein the conductive sections
comprise:
two conductive sections each having a current of (√2/3)i2, located along the cylindrical distribution, respectively, at θ ≈ 0° and θ ≈ 180°;
two conductive sections each having a current of (-√2/3)i2, located along the cylindrical distribution, respectively, at θ ≈ -90° and θ ≈ 90°;
two conductive sections each having a current of (√1/3)i2, located along the cylindrical distribution, respectively, at θ ≈ -45° and θ ≈ 135°;
and
two conductive sections each having a current of (-√1/3)i2, located along the cylindrical distribution, respectively, at θ ≈ 45° and θ ≈ - 135°.
11. A method of performing a nuclear magnetic resonance magic angle spinning (MAS) spectroscopic
analysis of a sample in a rotatable sample container, the method comprising:
locating the sample container in a stator;
applying a static magnetic field which influences the sample, the static magnetic
field being oriented in the z-direction; and
rotating the sample container about a rotation axis having an angle relative to the
z-direction of substantially θm = cos-1 √1/3;
characterized by
applying a gradient magnetic field with a gradient magnetic field generator comprising
a plurality of straight line conductive sections each of which contributes to the
gradient field when one of a plurality of respective predetermined currents is passed
through it, the straight line conductive sections being parallel to one another and
pependicular to a plane containing said rotation axis, wherein said predetermined
currents generate a gradient field wherein the field strength of the gradient field
in the z-direction changes linearly along said rotation axis.
12. The method of claim 11, wherein said predetermined currents are direct (DC) currents.
13. A method according to anyone of the claims 11 through 12 further comprising arranging
the conductive sections in a cylindrical distribution substantially parallel to and
equidistant from an axis passing through the sample, wherein the axis passes through
the center of the sample volume, and wherein a normalized magnitude of each of said
predetermined currents is substantially equal to cos 2 (θ - 45° + θm/2), where θ is an angular position of a given conductive section relative to the
z-direction.
14. A method according to anyone of the claims 11 through 13 further comprising providing
the conductive sections such that the each are part of a single conductor which is
wound about the stator.
15. A method according to anyone of the claims 11 through 14 further comprising providing
the stator with an ejection port for ejection ofthe sample container, and physically
supporting the conductive sections with the stator, the conductive sections being
positioned such that they do not inhibit ejection of the sample container through
the ejection port.
16. A method according to anyone of the claims 11 through 15, further comprising encompassing
the conductive segments with magnetic shielding.
1. Kernspinresonanz-Spektroskopie mit Rotation unter dem magischen Winkel (=MAS magic
angle spinning) zum Analysieren einer Probe, die in einem Probenbehälter enthalten
ist, wobei die Vorrichtung aufweist:
einen Generator für ein statisches Magnetfeld zum Erzeugen eines statischen Magnetfeldes,
welches die Probe beeinflusst, wobei das statische Magnetfeld in z-Richtung ausgerichtet
ist; und
einen Stator zur Aufnahme des Probenbehälters und Ermöglichen einer Rotation des Probenbehälters
in diesem um eine Rotationsachse mit einem Winkel relativ zur z-Richtung von im Wesentlichen
θm= cos-1 √1/3;
gekennzeichnet durch
einen Gradienten-Magnetfeld-Generator mit einer Vielzahl von geradlinigen, leitenden
Abschnitten, die jeweils zu einem Gradientenfeld beitragen, wenn einer aus einer Vielzahl
von entsprechenden, vorbestimmten Strömen
durch dieses fließt, wobei die geradlinigen, leitenden Abschnitte parallel zueinander und
senkrecht zu einer die Rotationsachse und die Hauptmagnetfeldrichtung enthaltenden
Ebene sind, wobei die vorbestimmten Ströme ein Gradientenfeld erzeugen und die Feldstärke
des Gradientenfelds in z-Richtung sich linear entlang der Rotationsachse ändert.
2. Vorrichtung nach Anspruch 1, wobei die vorbestimmten Ströme Gleichströme (DC) sind.
3. Vorrichtung nach Anspruch 1 oder 2, wobei die leitenden Abschnitte alle Teil eines
einzigen Leiters sind.
4. Vorrichtung nach Anspruch 3, wobei der einzige Leiter um den Stator gewickelt ist.
5. Vorrichtung nach einem der vorhergehenden Ansprüche, wobei die leitenden Abschnitte
in einer zylindrischen Verteilung so angeordnet sind, dass sie im Wesentlichen parallel
zu und in gleichen Abständen von einer Achse, die durch die Probe verläuft, angeordnet
sind, wobei die Achse durch den Mittelpunkt des Probenvolumens verläuft und wobei
eine normierte Größe jedes der vorbestimmten Ströme im Wesentlichen gleich cos 2(θ-45°+θm/2) ist, wobei θ eine Winkelposition eines gegebenen leitenden Abschnitts relativ
zur z-Richtung ist.
6. Vorrichtung nach einem der vorhergehenden Ansprüche, welche weiterhin eine magnetische
Abschirmung aufweist, die die leitenden Segmente umgibt.
7. Vorrichtung nach einem der vorhergehenden Ansprüche, wobei die leitenden Abschnitte
körperlich von dem Stator gestützt sind und der Stator weiterhin eine Auswurföffnung
aufweist, durch welche der Probenbehälter von dem Stator ausgeworfen werden kann,
wobei die leitenden Abschnitte so angeordnet sind, dass sie den Auswurf des Probenbehälters
durch die Probenöffnung nicht blockieren.
8. Vorrichtung nach Anspruch 7, wobei der vorbestimmte Strom durch jeden der leitenden
Abschnitte nicht mit der Rotation der Probe synchronisiert ist.
9. Vorrichtung nach einem der vorhergehenden Ansprüche, wobei die leitenden Abschnitte
zwei leitende Abschnitte umfassen, die jeweils einen Strom von i1 haben und entlang der zylindrischen Verteilung bei θ ≈ 17,65° bzw. θ ≈ 197,65° angeordnet
sind, und zwei leitende Abschnitte, die jeweils einen Strom von -i1 haben und entlang der zylindrischen Verteilung bei θ ≈ 107,65° bzw. θ ≈ 287,65° angeordnet
sind.
10. Vorrichtung nach einem der vorhergehenden Ansprüche, wobei die leitenden Abschnitte
aufweisen:
zwei leitende Abschnitte, die jeweils einen Strom von (√2/3)i2 haben und entlang der zylindrischen Verteilung bei θ ≈ 0° bzw. θ ≈ 180° angeordnet
sind;
zwei leitende Abschnitte, die jeweils einen Strom von (-√2/3)i2 haben und entlang der zylindrischen Verteilung bei θ ≈ -90° bzw. θ ≈ 90° angeordnet
sind;
zwei leitende Abschnitte, die jeweils einen Strom von (√1/3)i2 haben und entlang der zylindrischen Verteilung bei θ ≈ -45° bzw. θ ≈ 135° angeordnet
sind; und
zwei leitende Abschnitte, die jeweils einen Strom von (-√1/3)i2 haben und entlang der zylindrischen Verteilung bei θ ≈ 45° bzw. θ ≈ -135° angeordnet
sind.
11. Verfahren zum Durchführen einer Kernspinresonanz-Spektroskopieanalyse mit Rotation
unter dem magischen Winkel (=MAS magic angle spinning) einer Probe in einem drehbaren
Probenbehälter, wobei das Verfahren aufweist:
Anordnen des Probenbehälters in einem Stator;
Anlegen eines statischen Magnetfeldes, welches die Probe beeinflusst, wobei das statische
Magnetfeld in z-Richtung ausgerichtet ist; und
Drehen des Probenbehälters um eine Rotationsachse mit einem Winkel relativ zur z-Richtung
von im Wesentlichen θm = cos-1 √1/3;
gekennzeichnet durch
Anlegen eines Gradienten-Magnetfelds mit einem Gradienten-Magnetfeld-Generator, der
eine Vielzahl von geradlinigen, leitenden Abschnitten aufweist, die jeweils zum Gradientenfeld
beitragen, wenn einer aus einer Vielzahl von entsprechenden vorbestimmten Strömen
durch dieses hindurchfließt, wobei die geradlinigen, leitenden Abschnitte zueinander parallel
und senkrecht zu einer die Rotationsachse enthaltenden Ebene sind, wobei die vorbestimmten
Ströme ein Gradientenfeld erzeugen, wobei die Feldstärke des Gradientenfelds in z-Richtung
sich linear entlang der Rotationsachse ändert.
12. Verfahren nach Anspruch 11, wobei die vorbestimmten Ströme Gleichströme (DC) sind.
13. Verfahren nach einem der Ansprüche 11 bis 12, das weiterhin aufweist: Anordnen der
leitenden Abschnitte in einer zylindrischen Verteilung im Wesentlichen parallel zu
und im gleichen Abstand von einer Achse, die durch die Probe verläuft, wobei die Achse
durch das Zentrum des Probenvolumens verläuft, und wobei eine normierte Größe jedes
der vorbestimmten Ströme im Wesentlichen gleich cos 2 (θ - 45° + θm/2) ist, wobei θ eine Winkelposition eines gegebenen leitenden Abschnitts relativ
zur z-Richtung ist.
14. Verfahren nach einem der Ansprüche 11 bis 13, das weiterhin das Bereitstellen der
leitenden Abschnitte aufweist, so dass diese jeweils Teil eines einzigen Leiters sind,
welcher um den Stator gewickelt ist.
15. Verfahren nach einem der Ansprüche 11 bis 14, das weiterhin aufweist:
Ausstatten des Stators mit einer Auswurföffnung zum Auswerfen des Probenbehälters
und zur körperlichen Unterstützung der leitenden Abschnitte mit dem Stator, wobei
die leitenden Abschnitte so angeordnet sind, dass sie den Auswurf des Probenbehälters
durch die Auswurföffnung nicht blockieren.
16. Verfahren nach einem der Ansprüche 11 bis 15, das weiterhin aufweist, dass die leitenden
Segmente mit einer magnetischen Abschirmung umgeben sind.
1. Spectroscopie par résonance magnétique nucléaire à rotation d'angle destinée à l'analyse
d'un échantillon contenu dans un conteneur à échantillon, l'appareil comprenant :
un générateur de champ magnétique statique destiné à la génération d'un champ magnétique
statique qui influence l'échantillon, le champ magnétique statique étant orienté selon
l'axe des z ; et
un stator destiné à loger le conteneur à échantillon et à permettre la rotation du
conteneur à échantillon dans celui-ci autour d'un axe de rotation faisant par rapport
à l'axe des z un angle de θm = cos-1 √1/3 sensiblement ;
caractérisé par
un générateur de gradient de champ magnétique comprenant une pluralité de sections
conductrices rectilignes dont chacune contribue à un champ de gradient lorsqu'un courant
d'une pluralité de courants prédéterminés respectifs circule à travers lui, les sections
conductrices rectilignes étant parallèles les unes par rapport aux autres et perpendiculaires
à un plan contenant ledit axe de rotation et la direction principale du champ magnétique,
dans lequel lesdits courants prédéterminés génèrent un champ de gradient, dans lequel
l'intensité de champ du champ de gradient selon l'axe des z varie linéairement le
long dudit axe de rotation.
2. Appareil selon la revendication 1, dans lequel lesdits courants prédéterminés sont
des courants continus (CC).
3. Appareil selon la revendication 1 ou 2, dans lequel les sections conductrices font
toutes partie d'un conducteur unique.
4. Appareil selon la revendication 3, dans lequel le conducteur unique est enroulé autour
du stator.
5. Appareil selon l'une quelconque des revendications précédentes, dans lequel les sections
conductrices sont agencées selon une répartition cylindrique de telle sorte qu'elles
se trouvent sensiblement parallèles à un axe passant à travers l'échantillon et équidistantes
de celui-ci, dans lequel l'axe passe à travers le centre du volume de l'échantillon,
et dans lequel une grandeur normalisée de chacun desdits courants prédéterminés est
sensiblement égale à cos 2(θ - 45° + θm/2), où θ est une position angulaire d'une section de conducteur donnée par rapport
à l'axe des z.
6. Appareil selon l'une quelconque des revendications précédentes, comprenant en outre
un blindage magnétique entourant les segments conducteurs.
7. Appareil selon l'une quelconque des revendications précédentes, dans lequel les sections
conductrices sont physiquement supportées par le stator, et le stator comprend en
outre un orifice d'éjection à travers lequel le conteneur à échantillon peut être
éjecté du stator, les sections conductrices étant positionnées de façon à ne pas entraver
l'éjection du conteneur à échantillon à travers l'orifice d'éjection.
8. Appareil selon la revendication 7, dans lequel le courant prédéterminé à travers chacune
des sections conductrices est non synchronisé avec la rotation de l'échantillon.
9. Appareil selon l'une quelconque des revendications précédentes, dans lequel les sections
conductrices comprennent deux sections conductrices, chacune ayant un courant de i1, placées le long de la répartition cylindrique, respectivement, à θ ≅ 17,65° et θ
≅ 197,65 °, et deux sections conductrices, chacune ayant un courant de -i1, placées le long de la répartition cylindrique, respectivement, à θ ≅ 107,65 ° et
θ ≅ 287,65 °.
10. Appareil selon l'une quelconque des revendications précédentes, dans lequel les sections
conductrices comprennent :
deux sections conductrices, chacune ayant un courant de (√2/3)i2, placées le long de la répartition cylindrique, respectivement, à θ ≅ 0 ° et θ ≅
180 ° ;
deux sections conductrices, chacune ayant un courant de (- √2/3)i2, placées le long de la répartition cylindrique, respectivement, à θ ≅ -90 ° et θ
≅ 90 ° ;
deux sections conductrices, chacune ayant un courant de (√1/3)i2, placées le long de la répartition cylindrique, respectivement, à θ ≅ -45 ° et θ
≅ 135 °, et
deux sections conductrices, chacune ayant un courant de (-√1/3)i2, placées le long de la répartition cylindrique, respectivement, à θ ≅ 45 ° et θ ≅
-135 °.
11. Procédé de réalisation d'une analyse spectroscopique par résonance magnétique nucléaire
à rotation d'angle d'un échantillon dans un conteneur à échantillon rotatif, le procédé
comprenant :
le placement du conteneur à échantillon dans un stator ;
l'application d'un champ magnétique statique qui influence l'échantillon, le champ
magnétique statique étant orienté selon l'axe des z ; et
la rotation du conteneur à échantillon autour d'un axe de rotation faisant par rapport
à l'axe des z un angle de θm = cos-1 √1/3 sensiblement ;
caractérisé par
l'application d'un gradient de champ magnétique avec un générateur de gradient
de champ magnétique comprenant une pluralité de sections conductrices rectilignes
dont chacune contribue à un champ de gradient lorsqu'un courant d'une pluralité de
courants prédéterminés respectifs circule à travers lui, les sections conductrices
rectilignes étant parallèles les unes par rapport aux autres et perpendiculaires à
un plan contenant ledit axe de rotation, dans lequel lesdits courants prédéterminés
génèrent un champ de gradient dans lequel l'intensité de champ du champ de gradient
selon l'axe des z varie linéairement le long dudit axe de rotation.
12. Procédé selon la revendication 11, dans lequel lesdits courants prédéterminés sont
des courants continus (CC).
13. Procédé selon l'une quelconque des revendications 11 à 12, comprenant en outre l'agencement
de sections conductrices selon une répartition cylindrique sensiblement parallèle
à un axe passant à travers l'échantillon et équidistante de celui-ci, dans lequel
l'axe passe à travers le centre du volume de l'échantillon, et dans lequel une grandeur
normalisée de chacun desdits courants prédéterminés est sensiblement égale à cos 2(θ
- 45° + θm/2), où θ est une position angulaire d'une section de conducteur donnée par rapport
à l'axe des z.
14. Procédé selon l'une quelconque des revendications 11 à 13, comprenant en outre de
pourvoir les sections conductrices de telle sorte que chacune fasse partie d'un conducteur
unique qui est enroulé autour du stator.
15. Procédé selon l'une quelconque des revendications 11 à 14, comprenant en outre de
pourvoir le stator avec un orifice d'éjection destiné à l'éjection du conteneur à
échantillon, et supportant physiquement les sections conductrices avec le stator,
les sections conductrices étant positionnées de façon à ne pas entraver l'éjection
du conteneur à échantillon à travers l'orifice d'éjection.
16. Procédé selon l'une quelconque des revendications 11 à 15, comprenant en outre l'entourage
des segments conducteurs avec un blindage magnétique.